The conversion capacity can drop reversibly as well as irreversibly. It has been observed that sulfur intakes into the catalytic converter reduce the conversion capacity thereof. These sulfur intakes are caused by the sulfur components of the fuel. The intakes are dependent upon temperature and can be again released during operation of the engine. In this connection, reference can be made to SAE Paper 750697. The sulfur content in the fuel can fluctuate in dependence upon a fuel oil company (and/or the particular state in the United States of America). The fluctuation can be between 15 ppm and 1,000 ppm. It is therefore of interest not to deteriorate the exhaust gas notwithstanding an operation with fuel having a large sulfur content.